This invention relates to a gradient index optical waveguide having an optical refractive index profile.
The propagation of light waves in optical waveguides is governed by laws of physics similar to those that govern microwave propagation and therefore can be studied in terms of modes, each of which has its own propagation and electromagnetic field characteristics. Single mode waveguides are advantageous in that they are capable of propagating optical signals with very low dispersion, but due to the low numerical aperture and/or small core size of such fibers, it is difficult to efficiently inject optical signals into these waveguides. Multimode waveguides have larger core diameters and/or larger numerical apertures than single mode waveguides. Multimode waveguides are therefore often the preferred medium for the transmission of optical signals since they can accept light from incoherent, broad spectral width sources such as light emitting diodes. However, thousands of modes propagate in multimode optical waveguides, each mode traveling at a slightly different group velocity. A short input pulse that is shared by many guided modes thus splits up into a sequence of pulses that arrive at the output end of the waveguide at different times. This type of pulse dispersion is the dominant cause of dispersion in typical multimode optical waveguides.
Optical waveguides initially consisted of a core of uniform refractive index surrounded by a layer of cladding material having a lower refractive index. In this type of prior art fiber the time required for the various modes to travel a given longitudinal distance along the waveguide increases as the mode order increases. The delay distortion in such a fiber, defined as the difference in the times it takes the fastest mode and the slowest mode to traverse a given longitudinal length, is very large. It has been recently recognized that optical waveguides, the cores of which have radially graded index profiles, exhibit significantly reduced pulse dispersion resulting from group velocity differences among modes. This dispersion reducing effect, which is discussed in the publication by D. Gloge et al, entitled "Multimode Theory of Graded-Core Fibers," published in the November 1973 issue of the Bell System Technical Journal, pp. 1563-1578, employs a radially graded, continuous index profile from a maximum value at the center of the core to a lower value at the core-cladding interface. The index distribution in this type of waveguide is given by the equation: EQU n(r)=n.sub.1 [1-2.DELTA.(r/a).sup.60 ].sup.1/2 for r.ltoreq.a (1)
where n.sub.1 is the refractive index at the center of the core, n.sub.2 is the refractive index of the fiber core at radius a, .DELTA.=(n.sub.1.sup.2 -n.sub.2.sup.2)/2n.sub.1.sup.2 and a is the core radius.
Initially it was thought that a parabolic gradient wherein .alpha.=2 was optimal. Recently variations from an .alpha.=2 gradient have proven useful. U.S. Pat. No. 3,904,268--Keck and Olshansky describes a particularly desirable value of .alpha. which reduces the dispersive properties of the core and cladding.
For some glass compositions, the refractive index squared is linearly proportional to the dopant concentrations. That is, the square of the index of refraction at the center of the core is equal to the square of the index of refraction at the radius a plus a term which is linearly related to the concentration of the dopants. This can be expressed as: ##EQU1## where N is the number of dopants, C.sub.i is the concentration of the i.sup.th dopant glass and the P.sub.i are .lambda.-dependent proportionality factors. In such a case, the refractive index profile given by Eq. (2) can be fabricated by varying the dopants as: EQU C.sub.i (r)=C.sub.i.sup..degree. +C.sub.i.sup.1 (r/a).sup.2 ( 3)
However, often there is not a linear relationship between refractive index squared and dopant concentration. Multicomponent glass forming compounds which have been recently used in the fabrication of optical waveguides quite often do not have the linear relationship between concentration and refractive index squared.
The present invention provides a method of producing the desired gradient index profile from multicomponent glasses of any specified composition.